Development of microwave gas sensors

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Abstract

This work presents a novel approach in gas detection by an original method of microwave transduction. The design of the sensor includes a coplanar grounded wave guide with a gas sensing material to study its sensitivity to ammonia in argon flux.

The sensing material can play the role of the substrate or can be deposited as a thin layer on a microstrip structure used in electronics. Submitted to an electromagnetic excitation in microwave energies, the sensor response in the presence of a gas results in a specific modification of the reflected wave (real and imaginary parts). The goals of this study include an examination of the form of the sensitive material and its influence on the response of the microwave gas sensor. Two cases are considered: bulk or thin layer. In bulk case, the material plays the role of the substrate of the microstrip structure. In the second case, a thin layer is deposited on the sensor. We showed how, in the presence of ammonia, the reflected wave is related to its concentration. The response to the material–gas interaction depends on the excitation frequency. The parameter used as the sensor response is the ratio of the reflected wave on the incident wave at each frequency. The study deals with the influence of molecular sensing materials (CoPc) on the response of the sensor in the presence of ammonia. All the measurement were carried out at room temperature.

Introduction

Atmospheric pollution is a complex problem due to the varied nature of the involved chemical species and of their interactions. The air quality control constitutes public health problems like in the big urban areas or even inside the buildings. The current analysers, with an high cost and requiring a regular maintenance, do not allow to multiply the places of measurement.

The chemical sensors that play a growing part in the monitoring of environment constitute a possible alternative [1]. Indeed, they produce information on the polluting gas concentration, with response times compatible with environmental monitoring – in the minute range, or shorter if necessary –, in respect of a low level of energy consumption and at a cost largely lower than traditional analysers. The recent contributions of micro-electronics lead to the development of increasingly small gas sensors, at reduced costs and matched to industry.

The different gaseous species can be classified in three main classes, as toxic gases (CO, SO2, NOx, O3, NH3), corrosive gases (Cl2, F2, HCl, HF) and explosive gases (hydrocarbons, nitrated compounds). The gaseous species are detected by portable techniques based on indirect measurements, which depend on the variation of sensing materials physical properties. Among target volatil organic compounds (VOC) appear two first families, the sulfur compounds of the mercaptans type, RSH, and the amines, RNH2. A particular case is ammonia (NH3), for which new tools are needed, particularly for industrial discharge analysis. Indeed, liquid ammonia is used as a cryogenic liquid, particulary in cold rooms and more generally in the chain of food conservation, to replace freons. It is also produced in the industrial breddings by natural processing of animals excrements. In Europe and in the United States, it constitutes the main source of ammonia, between 70% and 90% of the estimated total emissions. Besides, when NH3 is used in chemical fertilizer industry, its concentration must be controlled, the threshold of 50 ppm in air corresponding to a warning level. There exists a need for reliable tools, able to deliver a measurement of the NH3 concentration, in real time, and also a need of leaks detectors.

In the past, the juxtaposition of non-selective sensors, in systems of recognition such as the electronic noses, was often proposed. However, it does not respond effectively to the needs of selectivity, because of drift in time of individual elements. According to a certain interest, particulary on the part of the academic researchers, mainly in Europe, the peak of sale of such systems was reached in 1997, followed by a sharp decline from 1998 [2].

The currently marketed ammonia sensors are of two types. The first are conductimetric sensors, generally using non-stoichiometric tin oxide (SnO2−x), presenting a longer lifespan. They can work from the ppm range to the percent range, under varied conditions and are available at low-costs, but their main drawback is they require a heating from 250° C [3], [4] up to 400° C [5], [6] and even more, and they are not selective. The main supplier is still the Figaro company, the historical manufacturer who markets the sensors with SnO2−x under the name of Taguchi sensor. The other commercial ammonia sensors, of the electrochemical type, have the same advantages and drawbacks than the other electrochemical sensors. They are sensitive in the ppm range, and are generally selective, but are much more expensive than the precedent ones, because their manufacture does not allow their production in great number by microelectronics techniques. Consequently, their replacement cost is high. They are moreover very sensitive to temperature variations. This is why ammonia is still the subject of many researches [7], [8], [9], [10], [11], [12].

The conductimetric transducers are mainly sensitive to redox active species that can accept or give electrons, which will induce a variation of the density of charge carriers and also sensitive to species able to modify the mobility of these carriers, and more generally the transport properties in sensing materials. We propose a new process, based on microwave transduction, which mainly measures the variation of the permittivity of sensing material consecutive to the adsorption of molecules on the surface of the sensing layer. The variation of the permittivity of the environment will be the main phenomenon in this technology whereas it only operates in the second order in the case of the conductimetric transducer. Here, the microwave transduction is applied to the detection of ammonia using metallophthalocyanines as sensing materials. Phthalocyanines and macrocycles are a family of sensing molecular materials used in the development of gas sensors, particulary with an accoustic transduction like surface acoustic wave (SAW)[13], [14], [15], [16], [17], but also with optical [18], [19], electrochemical [20], [21] or conductimetric [1], [22], [23], [24], [25], [26] transduction.

The microwave technology is based on the dielectric response of a material in the presence of a gas subjected to a microwave signal. It was proposed and validated recently with the non-stoichiometric tin dioxide as sensing material, aromatic molecules and a series of alcohols as analytes [27], [28], [29], [30]. This variation, which is specific to a gas and to a sensing material, depends on the working frequency. Using a vectorial network analyser, the sweep in a wide frequency range (30 MHz to 20 GHz) gives spectrum of this gas–material interaction. The geometry of the propagative structure is a type of microstrip, namely a grounded coplanar waveguide (GCPW), where a sensing material replaces the substrate or is deposited in thin layer on a glass substrate. So, the permittivity variation modifies the characteristic impedance and the propagation constant of the microstrip line that define the sensor. At each frequency, the wave reflected by the sensor is attenuated and out of phase compared to the incident wave, due to specific gas-sensing material interactions [31], [32], [33].

Section snippets

General principle of the microwave transduction

The principle of microwave transduction is based on the influence of the variation of dielectric properties of a sensing material in the presence of a gas when it is subjected to an electromagnetic wave in microwave frequency range (300 MHz to 300 GHz). The permittivity ɛ of a material is complex (ɛ = ɛ  ′′), which, in the range of microwaves, undergoes a strong decrease for its real part (ɛ′) whereas its imaginary (ɛ′′) part passes by an extremum (Fig. 1).

Any interaction with a gas should lead

Modus operandi

To evaluate the microwave transduction with CoPc as sensing material, two types of sensor were developed. In the first type, the bulk of the substrate was replaced by the sensing material. In the other case, the substrate was replaced by a glass slide covered by the sensing material film.

For the bulky form sensor, 150 mg of powder were compressed under primary vacuum at 10, 000 Lbs with steps of 2000 Lbs with the propagation lines (copper) for microwave measurement. The thickness of sensors was

Cobalt phthalocyanine under bulky form

The geometry and material characterisitics define, in this case, the resonance of the sensor. Fig. 4 represents the variation of the magnitude of Γ in dB for the sensor and indicate a resonant frequency (Fc) near 9.48 GHz and a quality factor (Q) of 74. In the following part, we studied the real and imaginary parts of the reflected coefficient.

The evolution of real and imaginary parts of ΔΓ are very different from each other (Fig. 5).

The real part of ΔΓ varies from 2 × 10−4 to 20 × 10−4, and the

Conclusion

This work represents the development of a new gas sensing process based on microwave transduction using a sensing material in electromagnetic structure. The precedent work [27] was adapted to metal oxides (SnO2) in presence of water vapour in a static regime. In our case, we evaluated this process in dynamic regime with ammonia in argon flux. The propagative structure used here is a grounded coplanar waveguide in two configurations. In the first case, the bulky substrate of the electromagnetic

Acknowledgements

The authors acknowledge the Conseil Régional de Bourgogne for funding through the program PARI SMT 08 IME-Région Bourgogne. Financial support from the European Union and the Conseil Régional de Bourgogne through the FABER program is gratefully acknowledged. The Université de Bourgogne is thanked for financial support and for a PhD grant (Guillaume Barochi). Europhtal is thanked for providing cobalt phthalocyanines.

Barochi Guillaume was born in 1985. He obtained a Bachelor of Science and a Master from the Université de Bourgogne (Dijon, France) in 2009. He is currently preparing his PhD under the supervision of Prof. M. Bouvet and Dr. J. Rossignol on new transducers for chemical sensors.

References (34)

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Barochi Guillaume was born in 1985. He obtained a Bachelor of Science and a Master from the Université de Bourgogne (Dijon, France) in 2009. He is currently preparing his PhD under the supervision of Prof. M. Bouvet and Dr. J. Rossignol on new transducers for chemical sensors.

Rossignol Jérôme was born in 1975. He obtained his PhD from the University Blaise Pascal (Clermont-Ferrand, France) in 2001 in physics plasma on theory and simulation of physical phenomena of cathodic arcs. He has been working for two years at the Humbolt University in Berlin (Germany) as a post-doctoral fellow. He is currently associate professor in electronics at the Université de Bourgogne (Dijon, France). His research activities are in the fields of microwave transduction and physics of plasmas.

Bouvet Marcel obtained his PhD from the University Pierre and Marie Curie (UPMC), in 1992, on electrical properties of phthalocyanines. He has been working mainly at the ESPCI (Paris), as associate professor, but also at the UPMC (Paris), and at the University of Connecticut (USA). He got a full professor position at the Université de Bourgogne (Dijon), in 2008. His research interests are in the field of molecular materials-based devices, among them sensors.

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